For years machine guns have been actuated by various mechanisms which utilize external power sources. The power source is usually an electric motor, which, through these mechanisms, performs basic weapon functions such as loading, locking, firing, extraction and advancement of the ammunition. One of the most common mechanisms of this type is that which is used in the mini-gun. This is a multiple barrel weapon, each barrel having its own separate bolt. The barrels and bolts rotate, which causes reciprocation of the bolts by engagement of a follower, on each bolt, with a serpentine cam track within the receiver. Another design utilizes a helical cam track in a rotating cylindrical drum to reciprocate the bolt. Still another design utilizes a crank with a connecting rod attached to the bolt. The most recent design uses a chain carried on four sprockets in a substantially rectangular path. The chain carries a pin which engages a slot on the bolt carrier and causes reciprocation of the bolt.
All of the above drive methods tend to be quite complicated. Most of them require expensive machining of difficult cam surfaces or achieve reliability through the use of expensive components.